That is, why not mix your own? I use a lot of Jo Sonja's artist paints. The thing I like about doing this is once you buy a few basic colours you can experiment and mix up your own special colour. You get a bit of experience with this and you know how to re-create it. Sure it won't be exactly the same, but that's exactly what I want.

I painted up a batch of 15mm Napoleonic Brits (for Spain) and I have every shade from a dark red to almost a pink (not quite). My rationale is that 'things stay stock for about 5 minutes after issue'. It's akin to (paraphrasing) 'no battle plan survives contact with the enemy'.

There are way too many variables. Consider this:

1 - sporadic cloud cover will cast different shadow across the battle field.

2 - differing times 'in the field' will create different shades from due to bleaching from the sun (and washing of clothes in the case of my Napoleonic Brits)

3 - Artificial Weathering (that done by the crew) will change the colour even more.

If I was to do Panzer Yellow in 15mm this is how I'd do it:

Undercoat very well in Black.
Dry brush with white
Divide the vehicles into three batches
first batch get Jo Sonja Yellow Oxide (P.Y.42) pretty well applied directly from the tube, although it is a bit dark.
second batch get Jo Sonja Yellow Oxide (P.Y42) with a bit of white mixed in. Enough so you can tell the difference between the colours. Then dry brush the first batch with the second batch's leftovers.
third batch get Jo Sonja Yellow Oxide (P.Y 42) with even more white mixed in. Then dry brush the second batch with the leftovers, and do a really really light dry brushing on the first batch.
add a bit more white and dry brush the third batch and then the second batch really really lightly
add a bit MORE white (so the stuff would be about 50:50 yellow oxide white by now) and do the really really light dry brush on the third batch.
do the detailing as you like - exhausts, tracks, etc, and weather additionally as you see fit. Make the tanks a real mess. Mud caked on. Even possibly get some putty to make a real mess of the things (yeah, it's disgusting to do this to $17.00 a packet of three, but it is more realistic) or if you don't want to use putty you can use the thick Jo Sonja artist paint direct from the tube without watering down any. It's thick stuff, and will dry like mud dries anyway.
last stage is to ink with Windsor&Newton 954 Nut Brown. I'd only do this stage if the figures had good detail to start with. If you do it with some cheaper models you bring out the bad casting etc, but good quality models bring out rivets and buttons, tracks etc really nicely.

Now, you only asked what colour is good for German Yellow, and I've gone into further detail. The reason I did was to show how "I'd" paint them using the paint that "I" use. And my philosophy in painting figures is for the most part it doesn't matter so long as you get it mostly right. Ask anyone who has been in the military, things don't stay the way they are meant to for very long when on campaign or exercise.

Hope this has been helpful.